The purpose of this grant is to define the pathogenesis of calcium-containing gallstones in order to develop a rationale for their prevention and medical treatment. Since calcium salts are a major constituent of all pigment stones, precipitation of calcium with various anions (bilirubinate, carbonate, phosphate, 'palmitate') is a clear requisite event in the initiation and growth of pigment gallstones. In addition, since all cholesterol stones have been found to have pigment stone centers, we postulate, as a first major hypothesis, that calcium precipitation is a critical event in the initiation of cholesterol gallstones, so that the latter should be considered a 2-part problem: (1) Precipitation of calcium salts to form a nidus, and (2) precipitation of cholesterol from its supersaturated state on this nidus. According to this view, calcium plays a crucial role in the fomation of both pigment and cholesterol gallstones, and retards or inhibits cholesterol stone dissolution by cheno- or ursodeoxycholic acids. Further elucidation of gallstone formation therefore requires definition and quantification of free Ca++ in bile, the reactive species for precipitation with 'calcium-sensitive' anions. The second major hypothesis is that bile acid anions are the major buffers for Ca++ in bile, and that free [Ca++] is largely governed by calcium complexation with bile salts. The third major hypothesis is that pH plays a key role in determining which of the calcium-sensitive anions will precipitate. The fourth hypothesis to be tested is that the biliary calcium originates primarily at the hepatic canaliculus, and that its secretion is coupled to that of bile acids. The fifth, and most important hypothesis is that gallstones are potentially preventable by prevention of calcium precipitation in bile. Any measure which will reduce free [Ca++] in bile, either by reducing calcium secretion, or by increasing Ca++ complexation, will reduce calcium lithogenicity. The proposed experiments will test these and other specific hypotheses, and will provide the first systematic studies of: (1) Free [Ca++] in bile and bile salt solutions; (2) Ca++-bile salt interactions; (3) The CO2 system in bile and bile salt solutions; and (4) Calcium carbonate and calcium bilirubinate solubilities in bile. The results should lead to new approaches to the medical management and prevention of gallstone disease.